| NILFS2 | |
| ------ | |
| NILFS2 is a log-structured file system (LFS) supporting continuous | |
| snapshotting. In addition to versioning capability of the entire file | |
| system, users can even restore files mistakenly overwritten or | |
| destroyed just a few seconds ago. Since NILFS2 can keep consistency | |
| like conventional LFS, it achieves quick recovery after system | |
| crashes. | |
| NILFS2 creates a number of checkpoints every few seconds or per | |
| synchronous write basis (unless there is no change). Users can select | |
| significant versions among continuously created checkpoints, and can | |
| change them into snapshots which will be preserved until they are | |
| changed back to checkpoints. | |
| There is no limit on the number of snapshots until the volume gets | |
| full. Each snapshot is mountable as a read-only file system | |
| concurrently with its writable mount, and this feature is convenient | |
| for online backup. | |
| The userland tools are included in nilfs-utils package, which is | |
| available from the following download page. At least "mkfs.nilfs2", | |
| "mount.nilfs2", "umount.nilfs2", and "nilfs_cleanerd" (so called | |
| cleaner or garbage collector) are required. Details on the tools are | |
| described in the man pages included in the package. | |
| Project web page: http://nilfs.sourceforge.net/ | |
| Download page: http://nilfs.sourceforge.net/en/download.html | |
| List info: http://vger.kernel.org/vger-lists.html#linux-nilfs | |
| Caveats | |
| ======= | |
| Features which NILFS2 does not support yet: | |
| - atime | |
| - extended attributes | |
| - POSIX ACLs | |
| - quotas | |
| - fsck | |
| - defragmentation | |
| Mount options | |
| ============= | |
| NILFS2 supports the following mount options: | |
| (*) == default | |
| barrier(*) This enables/disables the use of write barriers. This | |
| nobarrier requires an IO stack which can support barriers, and | |
| if nilfs gets an error on a barrier write, it will | |
| disable again with a warning. | |
| errors=continue Keep going on a filesystem error. | |
| errors=remount-ro(*) Remount the filesystem read-only on an error. | |
| errors=panic Panic and halt the machine if an error occurs. | |
| cp=n Specify the checkpoint-number of the snapshot to be | |
| mounted. Checkpoints and snapshots are listed by lscp | |
| user command. Only the checkpoints marked as snapshot | |
| are mountable with this option. Snapshot is read-only, | |
| so a read-only mount option must be specified together. | |
| order=relaxed(*) Apply relaxed order semantics that allows modified data | |
| blocks to be written to disk without making a | |
| checkpoint if no metadata update is going. This mode | |
| is equivalent to the ordered data mode of the ext3 | |
| filesystem except for the updates on data blocks still | |
| conserve atomicity. This will improve synchronous | |
| write performance for overwriting. | |
| order=strict Apply strict in-order semantics that preserves sequence | |
| of all file operations including overwriting of data | |
| blocks. That means, it is guaranteed that no | |
| overtaking of events occurs in the recovered file | |
| system after a crash. | |
| norecovery Disable recovery of the filesystem on mount. | |
| This disables every write access on the device for | |
| read-only mounts or snapshots. This option will fail | |
| for r/w mounts on an unclean volume. | |
| discard This enables/disables the use of discard/TRIM commands. | |
| nodiscard(*) The discard/TRIM commands are sent to the underlying | |
| block device when blocks are freed. This is useful | |
| for SSD devices and sparse/thinly-provisioned LUNs. | |
| Ioctls | |
| ====== | |
| There is some NILFS2 specific functionality which can be accessed by applications | |
| through the system call interfaces. The list of all NILFS2 specific ioctls are | |
| shown in the table below. | |
| Table of NILFS2 specific ioctls | |
| .............................................................................. | |
| Ioctl Description | |
| NILFS_IOCTL_CHANGE_CPMODE Change mode of given checkpoint between | |
| checkpoint and snapshot state. This ioctl is | |
| used in chcp and mkcp utilities. | |
| NILFS_IOCTL_DELETE_CHECKPOINT Remove checkpoint from NILFS2 file system. | |
| This ioctl is used in rmcp utility. | |
| NILFS_IOCTL_GET_CPINFO Return info about requested checkpoints. This | |
| ioctl is used in lscp utility and by | |
| nilfs_cleanerd daemon. | |
| NILFS_IOCTL_GET_CPSTAT Return checkpoints statistics. This ioctl is | |
| used by lscp, rmcp utilities and by | |
| nilfs_cleanerd daemon. | |
| NILFS_IOCTL_GET_SUINFO Return segment usage info about requested | |
| segments. This ioctl is used in lssu, | |
| nilfs_resize utilities and by nilfs_cleanerd | |
| daemon. | |
| NILFS_IOCTL_SET_SUINFO Modify segment usage info of requested | |
| segments. This ioctl is used by | |
| nilfs_cleanerd daemon to skip unnecessary | |
| cleaning operation of segments and reduce | |
| performance penalty or wear of flash device | |
| due to redundant move of in-use blocks. | |
| NILFS_IOCTL_GET_SUSTAT Return segment usage statistics. This ioctl | |
| is used in lssu, nilfs_resize utilities and | |
| by nilfs_cleanerd daemon. | |
| NILFS_IOCTL_GET_VINFO Return information on virtual block addresses. | |
| This ioctl is used by nilfs_cleanerd daemon. | |
| NILFS_IOCTL_GET_BDESCS Return information about descriptors of disk | |
| block numbers. This ioctl is used by | |
| nilfs_cleanerd daemon. | |
| NILFS_IOCTL_CLEAN_SEGMENTS Do garbage collection operation in the | |
| environment of requested parameters from | |
| userspace. This ioctl is used by | |
| nilfs_cleanerd daemon. | |
| NILFS_IOCTL_SYNC Make a checkpoint. This ioctl is used in | |
| mkcp utility. | |
| NILFS_IOCTL_RESIZE Resize NILFS2 volume. This ioctl is used | |
| by nilfs_resize utility. | |
| NILFS_IOCTL_SET_ALLOC_RANGE Define lower limit of segments in bytes and | |
| upper limit of segments in bytes. This ioctl | |
| is used by nilfs_resize utility. | |
| NILFS2 usage | |
| ============ | |
| To use nilfs2 as a local file system, simply: | |
| # mkfs -t nilfs2 /dev/block_device | |
| # mount -t nilfs2 /dev/block_device /dir | |
| This will also invoke the cleaner through the mount helper program | |
| (mount.nilfs2). | |
| Checkpoints and snapshots are managed by the following commands. | |
| Their manpages are included in the nilfs-utils package above. | |
| lscp list checkpoints or snapshots. | |
| mkcp make a checkpoint or a snapshot. | |
| chcp change an existing checkpoint to a snapshot or vice versa. | |
| rmcp invalidate specified checkpoint(s). | |
| To mount a snapshot, | |
| # mount -t nilfs2 -r -o cp=<cno> /dev/block_device /snap_dir | |
| where <cno> is the checkpoint number of the snapshot. | |
| To unmount the NILFS2 mount point or snapshot, simply: | |
| # umount /dir | |
| Then, the cleaner daemon is automatically shut down by the umount | |
| helper program (umount.nilfs2). | |
| Disk format | |
| =========== | |
| A nilfs2 volume is equally divided into a number of segments except | |
| for the super block (SB) and segment #0. A segment is the container | |
| of logs. Each log is composed of summary information blocks, payload | |
| blocks, and an optional super root block (SR): | |
| ______________________________________________________ | |
| | |SB| | Segment | Segment | Segment | ... | Segment | | | |
| |_|__|_|____0____|____1____|____2____|_____|____N____|_| | |
| 0 +1K +4K +8M +16M +24M +(8MB x N) | |
| . . (Typical offsets for 4KB-block) | |
| . . | |
| .______________________. | |
| | log | log |... | log | | |
| |__1__|__2__|____|__m__| | |
| . . | |
| . . | |
| . . | |
| .______________________________. | |
| | Summary | Payload blocks |SR| | |
| |_blocks__|_________________|__| | |
| The payload blocks are organized per file, and each file consists of | |
| data blocks and B-tree node blocks: | |
| |<--- File-A --->|<--- File-B --->| | |
| _______________________________________________________________ | |
| | Data blocks | B-tree blocks | Data blocks | B-tree blocks | ... | |
| _|_____________|_______________|_____________|_______________|_ | |
| Since only the modified blocks are written in the log, it may have | |
| files without data blocks or B-tree node blocks. | |
| The organization of the blocks is recorded in the summary information | |
| blocks, which contains a header structure (nilfs_segment_summary), per | |
| file structures (nilfs_finfo), and per block structures (nilfs_binfo): | |
| _________________________________________________________________________ | |
| | Summary | finfo | binfo | ... | binfo | finfo | binfo | ... | binfo |... | |
| |_blocks__|___A___|_(A,1)_|_____|(A,Na)_|___B___|_(B,1)_|_____|(B,Nb)_|___ | |
| The logs include regular files, directory files, symbolic link files | |
| and several meta data files. The mata data files are the files used | |
| to maintain file system meta data. The current version of NILFS2 uses | |
| the following meta data files: | |
| 1) Inode file (ifile) -- Stores on-disk inodes | |
| 2) Checkpoint file (cpfile) -- Stores checkpoints | |
| 3) Segment usage file (sufile) -- Stores allocation state of segments | |
| 4) Data address translation file -- Maps virtual block numbers to usual | |
| (DAT) block numbers. This file serves to | |
| make on-disk blocks relocatable. | |
| The following figure shows a typical organization of the logs: | |
| _________________________________________________________________________ | |
| | Summary | regular file | file | ... | ifile | cpfile | sufile | DAT |SR| | |
| |_blocks__|_or_directory_|_______|_____|_______|________|________|_____|__| | |
| To stride over segment boundaries, this sequence of files may be split | |
| into multiple logs. The sequence of logs that should be treated as | |
| logically one log, is delimited with flags marked in the segment | |
| summary. The recovery code of nilfs2 looks this boundary information | |
| to ensure atomicity of updates. | |
| The super root block is inserted for every checkpoints. It includes | |
| three special inodes, inodes for the DAT, cpfile, and sufile. Inodes | |
| of regular files, directories, symlinks and other special files, are | |
| included in the ifile. The inode of ifile itself is included in the | |
| corresponding checkpoint entry in the cpfile. Thus, the hierarchy | |
| among NILFS2 files can be depicted as follows: | |
| Super block (SB) | |
| | | |
| v | |
| Super root block (the latest cno=xx) | |
| |-- DAT | |
| |-- sufile | |
| `-- cpfile | |
| |-- ifile (cno=c1) | |
| |-- ifile (cno=c2) ---- file (ino=i1) | |
| : : |-- file (ino=i2) | |
| `-- ifile (cno=xx) |-- file (ino=i3) | |
| : : | |
| `-- file (ino=yy) | |
| ( regular file, directory, or symlink ) | |
| For detail on the format of each file, please see include/linux/nilfs2_fs.h. |